The only standard would be if your cabinet company only offers them in certain sizes. For us it didn't matter bc we built custom. A lot of people don't like towers, but we love ours. It's such great storage. Our vanity is 8'. Here are some of the things we did that I'm really happy about:

- Made the tower shallower than some of the ones I've seen in pictures. Ours is 9" deep not counting the door. The reason is that a lot of pictures show 12" deep towers and to me they sort of look like someone plunked a kitchen cabinet on the vanity. So we just made it shallower and the balance seems about right.

- Put an electrical outlet on either side, down low and turned sideways. To do this we have the electrical boxes inside and there is a panel (false drawer) at the bottom of the tower.

- Put an electrical outlet inside the tower for charging toothbrushes.

- Adjustable shelves inside.

- "Tied" to the ceiling. A lot of towers I saw in pictures were "floating" meaning they didn't connect to the ceiling. That looks weird to me and it's just a dust-collector. We have 10' ceilings so we had to decide how to handle that. I saw pictures of people building up a very large amount of trim work to tie to the ceiling but I didn't like that look either. If you have a 8' or 9' ceiling it's no big deal to tie it to the ceiling using trim. We ended up furring down above the vanity and tying to that with trim and putting small lights in the fur-down. I'm generally not a fan of fur-downs, but in this instance it was either that or use a very large amount of trim. And I'm an oddball in that I've never seen a vanity light that I like, so I wanted small LEDs anyway and this was a good place to put them.

- Put a small piece of extra countertop materials under the tower to prevent water damage that can occur over time. I saw this in a HOUZZ photo and read the comments from the designer. She showed some pictures of towers that had been damaged over the years from water that might not be wiped up right away. Especially since you are building this for teenagers, you should consider doing this. Just have your fabricator make a piece of countertop materials the size of the tower bottom, using the same edge profile as your counter. You put that on your countertop and the tower goes on top of that. It's like putting your tower on a cutting board. It's a but of insurance, especially with kids.

- A note about towel rings: We didn't put any. We just fold a towel by the sinks. If your vanity will be between 2 walls, you have 2 obvious places to put your towel rings. The other place you could put them is on the tower itself. I saw that in a HOUZZ picture, but then I wonder if that would interfere with the outlets on the sides. If you have 2 walls, outlets could go there, and towel rings on tower or visa versa.

Here's the HOUZZ picture showing the extra piece of marble under their tower:

Here's our tower:

If you go to HOUZZ and click on bathrooms and then put tower in the search bar, a lot of pictures will come up. Some are large, some small, some have open storage, others are closed. Some vanities have 2 towers, one on each end. If you look at a bunch of photos you'll be able to tell which features are most important to you.

Good luck!